Evolving high frequency communications technologies operating from around 27 GHz to 31 GHz are generally full-duplex in nature and are deployed in terrestrial microcells or configured to directly exchange data with low earth orbit satellites. Service providers typically divide the allocated transmit and receive frequency spectrums so that the transmitter in the satellite or base station operates in a lower, and therefore easier to work with, frequency block of around 27 GHz while the transmitter in the consumer's device operates at the higher frequency, e.g. 31 GHz.
FIG. 1 is a high level diagram of a conventional quadrature phase shift keying (QPSK) transmitter 10 used to transmit at around 31 GHz. The transmitter consists of a channel oscillator 11 operating at an intermediate frequency (IF), e.g., 1 GHz, which feeds a quadrature divider 12 that produces two output signals having a relative phase difference of 90 degrees. These signals are provided to separate bi-phase modulator switches 13, 14 which output either the original input signal, or the input signal phase shifted by 180 degrees, depending on the value of a respective input data bit 15, 16. Thus, the first modulator 13 will produce an output signal with a phase of 0 degrees or 180 degrees and the second modulator 14 will produce a signal with a phase of 90 degrees or 270 degrees, according to the value of the input data bits 15, 16. The output signals of the modulator switches are mixed with an in-phase combiner 17 to produce a four vector output signal 18.
The intermediate frequency QPSK signal 18 is input to a preamplifier 19 via a coaxial cable. The amplified signal is processed by an image filter 20 to reduce noise and then combined with a signal from a local high frequency oscillator 21 operating at the transmit frequency, e.g., 31 GHz, with a mixer 22. A local trap 23 may also be utilized to clip out emissions outside a particular bandwidth in order to comply with applicable government regulations. The final signal is then input to the transmit amplifier 24.
It is very difficult to economically generate enough power at 31 GHz to uplink a signal to a satellite or base station. A conventional solid state 31 GHz transmit amplifier is produced as a thin film integrated circuit using GaAs technology. Devices of this type which are powerful enough to produce a one-watt output signal typically cost several thousand dollars each. The high cost of the amplifier places the total cost of the transceiver unit, which includes a transmitter, a receiver, an antenna, and the equipment housing, well beyond the price range of most interested consumers.
Accordingly, it is an object of the invention to provide a high power transmit block for consumer satellite uplinks which may be inexpensively manufactured.
It is a further object of the invention to provide a high frequency transmitter which eliminates the need to internally generate a modulated signal at an intermediate frequency before producing the high frequency output signal.
Yet another object of the invention is to provide a high frequency QPSK transmitter block in which the signal modulation is implemented in a waveguide.